1. Field of the Invention
This invention relates broadly to diamond film manufacturing. More particularly, this invention relates to an apparatus and method for manufacturing free standing diamond films.
2. State of the Art
Diamond has exceptional hardness, thermal conductivity, electrical insulation and light transmission properties, and is therefore useful in various applications such as cutting tools, heat sinks, insulators, electronic substrate materials, etc. Natural diamond, however, is monocrystalline and limited in size and geometry. As a result, a number of techniques have recently been developed, such as high pressure high temperature deposition (HPHT) and chemical vapor deposition (CVD), for synthesizing and depositing diamond on substrates of various shapes, sizes and materials. The below discussion relates to CVD diamond film deposition.
Synthetic CVD diamond film can be deposited as a thin permanent coating on a substrate, such as on the wear surface of a tool or as an environmentally protective coating. Such films are generally referred to as thin films. Alternatively, a thicker diamond film can be deposited on a substrate (sometimes known in the art as a xe2x80x9cmandrelxe2x80x9d) and then removed, preferably intact, as a single xe2x80x9cfree standingxe2x80x9d piece for use in applications such as heat sinks, optical windows, and cutting tools. These free standing pieces are usually referred to as thick films.
In the manufacture of thick films, it is convenient to deposit the diamond on a substrate and then allow thermal stresses during cooling to detach the diamond from the substrate. This process eliminates the operation of removing the diamond from the substrate, and allows the substrate to be reused. In general, several considerations must be taken into account when manufacturing free standing diamond films by depositing synthetic diamond on a substrate. A relatively weak bond between the diamond and substrate is essential to ensure that the diamond will easily detach from the substrate. The substrate material and diamond generally have different coefficients of expansion, as well as different molecular and chemical structures, which affects the growth, adhesion, and smoothness of the diamond film. Other factors, such as surface preparation, and deposition parameters will also affect the growth and adherence of the synthetic diamond on the substrate.
Titanium nitride coated molybdenum and other materials having similar properties, such as titanium-zirconium-molybdenum alloys and tungsten, have traditionally been used as a substrate (mandrel) upon which synthetic diamond is to be deposited. These materials are chosen because of their temperature properties including coefficients of expansion, and their machinability. A layer of synthetic diamond can be deposited on a TiN coated molybdenum substrate, such as by CVD, and then released from the substrate after the desired thickness of diamond film is reached. The diamond is deposited on the substrate at a relatively high temperature and, as the diamond and the substrate cool after completion of the diamond deposition, the diamond is released from the substrate as a result of the difference in the coefficient of thermal expansion of the diamond and the substrate materials. Since the outer edges of the substrate cool faster than the inner portions of the substrate, several problems may arise during this procedure. Diamond film may detach from the substrate prematurely, resulting in an incomplete and defective diamond film. Also, upon release of the diamond film from the substrate after deposition, the diamond film tends to crack at the edges and/or break.
It is therefore an object of the invention to provide a substrate and method for making a free standing diamond film.
It is also an object of the invention to provide a substrate and method for making a free standing diamond film which allows for the more reliable and easy removal of the diamond film from the substrate.
It is a further object of the invention to provide a substrate for making a free standing diamond film which prevents the diamond film from releasing prematurely from the substrate while diamond is being deposited on the apparatus.
It is also an object of the invention to provide a substrate which reduces the cracking/breaking of a diamond film when the diamond releases and is subsequently removed from the substrate.
In accordance with the objects of the invention, which will be discussed in detail below, a mandrel substrate is provided having a surface with varying diamond adhesion properties. In general, it is preferable that the substrate have greater diamond adhesion properties on or near its outer edge, but it may also be desirable to provide an adhesion gradient along the surface of the substrate, depending on the type of diamond being manufactured.
In a first embodiment of the invention for forming a free-standing cup-shaped diamond film, the mandrel is a titanium nitride (TiN) coated molybdenum (Mo) substrate having a stepped solid cylindrical shape with a central mesa having a side wall, or flank. A band is etched or machined around the mesa perimeter on the side wall near the top surface of the mesa to expose the underlying molybdenum and form a second surface which bounds the TiN first surface. The exposed molybdenum exhibits a stronger adhesion to diamond than does the TiN. When the molybdenum band loses efficiency as a result of diamond material remaining in the molybdenum band after a diamond deposition procedure, a second strip of the TiN coating adjacent to the first strip may be machined to expose a second band of molybdenum.
In a second embodiment of the invention for forming a free standing diamond film wafer, the molybdenum band may be etched directly on the top surface of the mesa of the mandrel, resulting in a first circular surface of TiN surrounded by an outer ring of molybdenum forming a second surface. As in the first embodiment, additional adjacent molybdenum bands may be etched as desired after repeated use.
A third embodiment of the invention includes combining aspects of the first and second embodiments. Accordingly, a first molybdenum band is etched on the top of the mesa in a fashion similar to the second embodiment, and a second molybdenum band is etched on the side wall of the mesa as described in the first embodiment. If desired, the top surface and side surface bands may be contiguous. As with the first two embodiments, additional molybdenum bands may be etched to replace used ones.
According to a fourth embodiment of the invention, a separate detachable molybdenum foil, band, or wire that can either be fitted like a collar around the top portion of the mesa of the mandrel or laid in a shallow circular groove on the top or side surface of the mesa is provided. The molybdenum foil or wire can be replaced after each use with a new and clean molybdenum foil or wire, thereby dispensing with the need to further machine the mandrel to expose the underlying molybdenum substrate.
Additional embodiments of the invention include mandrels having a plurality of exposed molybdenum surface areas in the form of stripes or patches on the top surface of the mesa to provide a surface for diamond growth which has a diamond adhesion differential or diamond adhesion gradient.
In accord with the invention, the diamond adhesion properties of the mandrel substrate are influenced by the nature of the material on which the diamond film is deposited and by the relative roughness of the mandrel surface. As explained above, molybdenum has greater diamond adhesion properties that TiN. It will be noted, also, that rough surfaces have greater diamond adhesion properties than smooth surfaces. Therefore, the principles of the invention may also be achieved by varying the roughness/smoothness of the mandrel substrate to effect a diamond adhesion differential or diamond adhesion gradient.
According to the method of the invention, which relates closely to the apparatus, a mesa mandrel is prepared as described above to have a band of exposed molybdenum around the sidewall and a diamond film is deposited on the mandrel of the invention until a desired diamond film thickness is achieved. The mandrel and the diamond film are then cooled. Due to the different coefficients of thermal expansion of the diamond film and the mandrel, the mandrel tends to contract under the diamond and separate from the diamond film. The portion of the mandrel having increased diamond adhesion properties (e.g. the exposed molybdenum band in some cases) causes the diamond to crack in a controlled manner at the boundary of the more adherent and less adherent portions of the mandrel. This can result in a razor clean break in the diamond film around its perimeter.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.